1. Field of the Invention
The present invention relates to a method and an apparatus for manufacturing a silica glass-based optical fiber, and particularly to a method and an apparatus for manufacturing a silica-based optical fiber having bending resistance improved by imparting a residual compressive stress to the surface layer of a bare optical fiber.
2. Description of the Related Art
Generally, in a method for manufacturing a silica glass-based optical fiber, it is common to heat and melt an optical fiber preform made of silica-based glass in a drawing furnace, draw the optical fiber preform into a linear shape from the drawing furnace, cool and solidify it so as to produce a bare optical fiber, further, coat the bare optical fiber with a protective coating resin, pull the bare optical fiber using a take-up machine, and, further, wind the bare optical fiber around a bobbin. Being dependent on the fiber drawing speed, as a manufacturing apparatus applied to the manufacturing of the above silica glass-based optical fiber, it is common practice to use an apparatus as illustrated in FIG. 6 in a case in which the fiber drawing speed is low, and to use an apparatus as illustrated in FIG. 7 in a case in which the fiber drawing speed is high.
The manufacturing apparatus illustrated in FIG. 6 has a drawing furnace 14 for heating and melting an optical fiber preform 12 made of silica-based glass, a cooling zone 18 for cooling and solidifying a bare optical fiber 16 drawn into a linear shape from the drawing furnace 14 in the atmosphere, a coating device 20 for coating the cooled and solidified bare optical fiber 16 with a protective coating resin, a curing device 22 for curing the coating resin using the coating device, a take-up device 26, such as a capstan, for taking up an optical fiber 24 with the cured protective coating resin, and, finally, a winder, which is not shown in the figure, for winding the optical fiber 24.
Meanwhile, in the manufacturing apparatus illustrated in FIG. 7, a cooling device 18A is provided in the cooling zone 18 between the drawing furnace 14 and the coating device 20. The bare optical fiber 16 drawn into a linear shape from the drawing furnace 14 is cooled using the cooling device 18A. Generally, the cooling device 18A has a double-walled structure (jacket structure). In the cooling device 18A, the wall portions are cooled using a cooling medium, such as cooling water, and a cooling gas which has favorable thermal conductivity and has no adverse influence on the material of the bare optical fiber 16, for example, He gas, is introduced into an inside space (cooling space) through which the bare optical fiber 16 passes, so as to cool the bare optical fiber 16.
An optical fiber is manufactured in the following manner using the above optical fiber manufacturing apparatus. The optical fiber preform (silica-based glass preform) 12, which serves as the raw material of the bare optical fiber, is heated at a high temperature of 2000° C. or higher in the drawing furnace 14 so as to be melted. The high-temperature optical fiber preform 12 is drawn downward from the bottom of the drawing furnace 14 while being elongated to be the bare optical fiber 16. The glass material of the bare optical fiber 16 is solidified and cooled (cooling in the atmosphere or cooling using the cooling device 18A) in the cooling zone 18 to a temperature at which the bare optical fiber 16 can be coated with a resin. The bare optical fiber 16 cooled up to the necessary temperature is coated with an uncured resin in the coating device 20 for protection. Further, the coated resin is cured in the curing device 22, thereby forming the optical fiber 24 having a protective coating layer. In the curing of the resin, appropriate means, such as heating, curing, or ultraviolet curing, can be used depending on the type of the resin. The optical fiber 24 is taken up at a predetermined speed through a turn pulley 28 using the take-up device 26. Further, generally, the optical fiber 16 is wound using a winding device, such as a bobbin, through a dancer roller which is not shown in the figure.
In recent years, the development of an optical fiber having excellent bending loss properties has been underway; that is, an optical fiber exhibiting a small bending loss even when bent at a small bend radius is imparted. In an apparatus in which an optical fiber is used, there are cases in which the optical fiber is temporarily bent at a small bend radius of 5 mmφ or less. Meanwhile, in a case in which the optical fiber is bent in a loop shape, a coil shape or other bent shape, a tensile stress is generated on the outside of the bent portion (outside bend). As the bend radius decreases, the tensile stress being exerted to the outside of the bent portion of the optical fiber increases accordingly.
When the tensile stress in the bent portion of the optical fiber exceeds the fracture limit of the material, the optical fiber fractures at the bent portion. In addition, even in a case in which the tensile stress in the bent portion does not exceed the fracture limit of the material, when the optical fiber stays bent for a long period of time or is repeatedly bent, the optical fiber fractures in the bent portion due to fatigue fracture. Therefore, if it is assumed that the optical fiber is used while being bent at a small bend radius, the optical fiber needs to be excellent in terms of bending resistance. Since the fracture in the bent portion results mainly from the tensile stress being exerted to the outside of the bent portion as described above, the relaxation of the tensile stress on the outside of the bent portion, which is generated when the optical fiber is bent, can be considered for improving the bending resistance.
A method for improving the bending resistance from the above viewpoint has already been proposed in Japanese Unexamined Patent Application, First Publication No. H1-301531 (hereinafter referred to as PTL 1). PTL 1 discloses that, in a process in which a glass preform is softened, melted and continuously drawn into a fiber (fibrillation), after a bare optical fiber is solidified, the surface of the bare optical fiber is reheated while a tensile force for fiber-drawing is applied to the optical fiber so as to soften and melt only the surface layer (therefore the center portions stay solidified), and, subsequently, the surface layer is re-solidified while the tensile force is applied. In this method, it is possible to eliminate fine cracks on the surface by remelting only the surface layer of the bare optical fiber which has been fully solidified temporarily. Further, a strain generated in the bare optical fiber due to the tensile force for fiber drawing (tensile strain) is relaxed in the surface layer of the bare optical fiber so that a compressive stress remains in the surface layer when the tensile force is released afterward. In such a manner, when an optical fiber having a compressive stress remaining on the surface is bent, the tensile stress generated in the surface layer on the outside of the bent portion is relaxed or canceled out by the residual compressive stress. Therefore, even in a case in which the bend radius is small, the tensile stress being exerted to the outside of the bent portion substantially decreases, and, consequently, cracking or fracturing due to bending is not easily caused.
However, as a result of carrying out experiments regarding the method disclosed in PTL 1, the present inventors found the following problems.
That is, PTL 1 simply proposes that, after the bare optical fiber drawn from the drawing furnace is cooled so as to be fully solidified, the surface of the bare optical fiber is reheated in the presence of a tensile force so as to remelt the surface layer. In particular, regarding the temperature of the bare optical fiber when the surface of the bare optical fiber is reheated so as to remelt the surface layer, PTL 1 only describes an example in which the reheating of the bare optical fiber cooled to approximately 200° C. The inventors found that, when the surface layer is reheated in the step in which the bare optical fiber is fully solidified, and is re-solidified according to the method described in PTL 1, there are many cases in which the residual compressive stress cannot be stably imparted to the surface layer of the optical fiber in the bare fiber portion after the release of the tensile force. Particularly, it was found that, in a case in which the surface layer is reheated and re-solidified in a step in which the bare optical fiber is cooled up to the temperature, that is, approximately 200° C. as described in an example of PTL 1, the residual compressive stress is not imparted to the surface layer of the optical fiber in the bare fiber portion after the release of the tensile force, or the residual compressive stress is not uniformly imparted in the longitudinal direction throughout the optical fiber such that the bending resistance cannot be reliably and stably improved.
The present invention has been made in consideration of the above circumstances, and an object of the present invention is to provide a method and an apparatus for manufacturing an optical fiber in which a residual compressive stress is reliably and stably imparted to the surface layer of the bare optical fiber portion so that the bending resistance is reliably and stably excellent.